ion propulsion

Ion propulsion is a form of electric space propulsion in which ions are accelerated by an electrostatic
field to produce a high-speed (typically about 30 km/s) exhaust. An ion
engine has a high specific impulse (making it very fuel-efficient) but a very low thrust.
Therefore, it is useless in the atmosphere or as a launch vehicle, but extremely
useful in space where a small amount of thrust over a long period can result
in a big difference in velocity. This makes an ion engine particularly useful
for two applications: (1) as a final thruster to nudge a satellite into
a higher orbit and or for orbital maneuvering or station-keeping, and (2)
as a means of propelling deep-space probes by thrusting over a period of
months to provide a high final velocity. The source of electrical energy
for an ion engine can be either solar (see solar-electric
propulsion) or nuclear (see nuclear-electric
propulsion).

Two types of ion propulsion have been investigated in depth over the past
few decades: electron bombardment
thrusters and contact ion thrusters.
Of these, the latter remains in the research stage while the former has
already been used on a number of spacecraft. Specifically, the variety of
electron bombardment thruster known as XIPS (a Hughes/Boeing product) is used for station-keeping by some geosynchronous satellites, while the NSTAR ion engine (developed by NASA and Hughes) propelled the Deep
Space 1 interplanetary probe.

One of the most promising new developments in ion propulsion is the DS4G
(dual-stage 4-grid) ion engine
, developed by the European Space Agency and a group at the Australian National
University. This was first tested by ESA in 2005. The DS4G thruster achieves
much higher voltages to be used than previously thought possible, resulting
in a more powerful post acceleration of the extracted ions. The thruster
was tested in a large space simulation chamber in the ESA Technology centre
in the Netherlands at a remarkable 30,000 V and produced an ion exhaust
plume that travelled at 210 km/s – over four times faster than state-of-the-art
ion engine designs achieve.

History of ion propulsion

A NASA engineer prepares an early ion engine for
a vacuum chamber test in 1959. Lined up at right are the major electrical
parts.

Among the most difficult challenges in the early development of ion engines
was proving that injecting electrons could neutralize an ion beam. Continually
spewing positively charged ions will leave a spacecraft with a negative
charge so great that the ions are attracted back to the spacecraft. The
solution is an electron gun that dumps the electrons into the ion stream,
thus neutralizing both spacecraft and exhaust. But the beam's interaction
with the walls of even a large vacuum chamber makes it very difficult to
conduct meaningful beam neutralization experiments on Earth. These uncertainties
led to considerations for flight testing electric engines. Another challenge
of electronic propulsion involved developing an efficient technique to produce
ions. Working at NASA's Lewis, Harold Kaufman invented an electron-bombardment
technique to ionize mercury atoms.
At NASA/Marshall, a process was under development whereby cesium atoms would become ionized upon contact with a hot tungsten or rhenium surface. Marshall's major development in electrical propulsion
centered, however, on a 30-kilowatt ion engine development contract, initiated
in September 1960 with Hughes Research Laboratory in Malibu, California.
At first, Marshall directed Hughes to design a laboratory model of an ion
engine. The 0.01 lb.-thrust model would be followed by the development of
a 0.1 lb.-thrust engine. Marshall later modified the Hughes contract to
include a flight test model ion engine, primarily to determine whether a
beam neutralization problem existed in space.

On August 1, 1961, NASA awarded a contract to the Astro-Electronics Division
of RCA to design and build a payload capsule for flight-testing electric
propulsion engines. The program called for seven capsules, three for ground
tests and four for actual flight tests. Each capsule was expected to carry
two electric engines. The first was expected to carry one cesium-fueled
ion-engine representing Stuhlinger's design with the Hughes engine. The
second was expected to carry one mercury-fueled ion engine representing
Kaufman's design with the Lewis engine. Plans called for the engines to
operate from 1 to 2 kW of power. Hughes demonstrated an ion engine on September
27, 1961, at its research laboratories in Malibu. Stuhlinger was among those
on hand to greet the scientific and technical writers who attended the event.

Ion propulsion in science fiction

Frequent mention of ion propulsion has been made in works of science fiction
for several decades. It was featured, for example, in a September 1968 episode
of Star Trek called "Spock's Brain," in
which invaders steal Spock's brain and flee in an ion-powered spacecraft.